A Mezuzah can last for many years when well protected, however, it does need to be inspected. Temperature and weather changes as well as age may cause the ink to fade and crack, thereby invalidating the Mezuzah. Each Mezuzah should be examined by a reliable scribe at least twice every seven years.
Outdoor Mezuzah’s should have a waterproof case, and extra attention should be given to any mezuzah that is on an external wall as well. Remember, a well protected mezuzah means a well protected home!